1. Field of the Invention
The invention relates to biological produce preservation by means of freezing and/or Freeze Drying said produces. More specifically, the invention relates to an apparatus able to perform consecutively, independently and, or simultaneously the processes of Individually Quick Freezing (IQF) and Freeze Drying (FD), and the methods to develop such processes.
Furthermore, the invention can be applied to other non biological Freeze Drying processes, such as pigments production, purification of minerals, and many others.
2. Description of the Related Art
A brief description of biologic produce preservation leads to consider the inhibition of the following processes, which have the main responsibility of their deterioration:                Microbial activity        Enzymatic activity        Chemical reactionsFor those purposes, the main used methods are        Refrigerating: Lowering the temperature to a few degrees centigrade over zero diminishes all biologic and chemical processes. It is the less invasive method. Is useful to preserve produces for a short time (bacteria, lettuce, meat, etc.), and in some instances for medium storage time (vaccines, serum, apples, potatoes, etc.)        Freezing: The produce's temperature is lowered below their freezing point, which corresponds to some degrees centigrade below zero. (Depending on produces' sugar and/or salt content, their freezing point can be from a couple to fifteen or twenty centigrade below zero.) Freezing is the best way for medium and long storage time, providing freezing is performed at sufficient velocity to avoid cellular damage and a very low temperature is maintained permanently.        Freeze Drying: Is the second best way for medium and long storage time, provided good freezing and packaging be performed, and the best way to preserve produces at room temperature.        Drying: Natural drying may be the earliest preservation method in human history, good for cereals and some fruits. Accelerated methods have been developed comprising Spray Drying, Flake Drying, Fluidized Bed Drying, etc.        Thermal treating: Mainly Pasteurization and Sterilization, very used for milk, pouched or canned food, etc.        Water activity reduction: Achieved by the addition of important quantities of sugar (for sweets, marmalades, etc.) or salt.        Radiation: Sterilization is produced by the exposition to radioactivity, mainly gamma rays.From all the above mentioned methods, the first three will be more detailed for their relation with the present invention.2.1. Refrigeration        
A first classification can be made between methods where indirect or direct contact between the refrigerating fluid and the produce is made.
The first instance is the most frequent, where the typical refrigeration cycle comprises three subsystems: an evaporator (normally a coil where a liquid is expanded and evaporated while absorbing its latent heat); a compressor, where the vapor is compressed and its temperature is raised by means of the compression; and a condenser, (normally another coil where the vapor looses its latent heat and condenses to liquid which goes again to the evaporator).
Different substances as Ammonia, Freon or Hydrocarbons are used as refrigerating fluids, depending on the desired refrigerating temperatures. An intermediary fluid, generally air, is cooled by means of contact with the evaporator coil, and it moves by natural or forced convection through the produces to be cooled.
In the direct contact method, the refrigerating fluid (In many cases water) is cooled and irrigated over the produce. This method, known as Hydro cooling, is very used immediately after harvesting lettuce or other vegetables. The water is cooled by means of its own evaporation, without need of a refrigerating cycle as in the indirect contact method.
Another alternative of the direct contact method consists in irrigating the produces with water at room temperature and cooling the water and the produces at the same time by means of water evaporation produced by vacuum inside the refrigeration chamber.
A very accurate way to control the temperature of water and of the irrigated produces is to control the pressure, as there is an exact relation between it and the equilibrium temperature of the liquid-vapor system. This method is known as Vacuum Cooling and constitutes one of the most significant antecedents to the present invention. U.S. Pat. Nos. 5,992,169 and 5,386,703 refers to apparatus and methods for cooling vegetables by means of water chilled by vacuum, and/or by means of evaporation of water added to the produce.
U.S. Pat. Nos. 5,375,431; 5,386,703; 4,576,014; 3,844,132 amongst others, describe apparatus comprising a vacuum chamber for receiving produce, a vacuum pump, a refrigeration system for collecting evaporated water and a pump to spray the water onto the produce. The vacuum pump reduces the pressure within the chamber to sub atmospheric level, causing evaporation of moisture from the produce. This evaporation removes heat from the produce, reducing its temperature. Water vapor formed by such evaporation condenses on cooling coils positioned above the produce. These refrigerated coils preferably condense and collect as much water as is feasible to prevent the water from reaching the vacuum pump. This water is collected and directed to a reservoir below the produce. The collected water, in preferred embodiments, is at a temperature in the range of about 1 to 4 degrees Centigrade.
Additionally, a water recirculating system can utilize water from the reservoir at the bottom of the vacuum chamber, and spray it over the produce, for further cooling effect, this reservoir water may be passed through a filtration device utilizing non residual free radical chemical methods of filtration or ultraviolet light to reduce the micro biotic load and insure freshness.
All these systems are limited to final temperatures over zero ° C.
2.2. Freezing
In a biologic produce, water is present in many forms; it can be free on the surface; adsorbed in it; as a solvent in the intercellular space; as a solvent inside the cells; combined with other substances; constituting sugar crystals; etc.
All those different ways in which water is found have different freezing points, so that full freezing is reached when all those solutions are frozen. But in industrial practice a substance is considered frozen when most forms of water are frozen, even if a small quantity of water remains in liquid state.
In food industry produces are frequently considered frozen at −18° C., even when for lamb meat at this temperature only 88% of the water is frozen, 91% for fish and 93% for egg albumin.
A very important phenomenon in freezing biological produces is constituted by the size of the ice crystals because, if the size is comparable to cellular size, breakage of cells occurs, the produce structure is damaged, intracellular fluids are lost when the produce defrosts, denaturalization of proteins can be produced, etc.
Ice crystal size is controlled by the freezing velocity, ranging from a very small size when freezing occurs in ten to twenty seconds, up to a millimeter or more if freezing time is of around one hour or more.
As in most freezing methods heat has to be extracted from the produce, the freezing time increases with the produce's density and mass, and the shape factor volume/surface, and decreases with produce's thermal conductivity.
In consequence, higher velocities are achieved when small size produces are individually frozen, and an industrial standard has been developed, identified with the acronym IQF (for Individually Quick Freezing, or Individually Quick Frozen).
IQF is referred to as a freezing process occurring In a short time (In the range of around one half of a minute) and to the produces therein obtained. In this short freezing time, ice crystals have no time to grow, and a great quantity of small size crystals are obtained, homogeneously distributed in the inter and intra cellular space.
In this condition crystals size is very small (they are not visible to the optical microscope), and no rupture of the cells is produced. For the same reason, there is no time for water to migrate from the intra to the intercellular space, so no increase in salt concentration inside the cells is produced, and no denaturalization of cellular proteins occurs.
To achieve such freezing velocity, cryogenic methods are used, where the produce, in small pieces passes trough a rain of liquid nitrogen or liquid carbonic anhydride, where it is quickly frozen.
Even with the inconvenient of the high energetic cost of this process (around 1.5 Kg of liquid nitrogen is needed to freeze 1 Kg of vegetables, and after being frozen, the produces have to be stored at around −20° C.), the quality of the produces is so good (it is the best way to preserve biologic produces) that IQF usage is growing all over the world. The following table shows typical freezing times of different methods used for some foods:
Freezing timeFreezing method[minutes]FoodAir (natural convection)  180–4,000Plates25–75Fish, vegetablesAir (forced convection)15–20Beans, (bulk)Spiral tunnel12–19Hamburger, fishFluidized bed 3–15Beans, (individual)Scratch surface0.3–0.5Ice creamCryogenic (Liquid Nitrogen)0.2–5  Hamburger, Sea food,(IQF)
U.S. Pat. Nos. 5,079,932 4,928,495; 4,901,535; 4,759,191; 4,736,599 and 4,250,720 describe method and or apparatus to freeze produces by means of vacuum applied, but these patents do not take into account the size of ice crystals or the freezing velocity.
2.3. FD
Freeze Drying is the second best way to preserve biologic produces, and the best way to do it without the need to store the produces at low temperature. In this process, the produces are first frozen and after they are dried, in most cases by means of vacuum below 2 Torr applied to them, thus making water contained in the produce to sublimate.
When most of the water is eliminated, the produces' temperature is raised, and the residual humidity is lost. The resulting produce structure is a sponge, formed by the solid part of its cells and fibers. As the water concentration is very low, no microbial activity is possible and, if oxidation is avoided by means of storing the produce in an inert atmosphere, it can be preserved many years at room temperature, without biochemical changes.
When the produce has to be used, it can be easily rehydrated, by the sole addition of water, and the original properties are recuperated. (Ferments, bacteria, antibiotics, blood plasma, food and beverages are preserved by this way).
Freeze Drying has no need of a low storage temperature, but since it adds the cost of water sublimation to the IQF costs, its usage has up to now been restricted to high valued produces, i.e. pharmaceuticals, astronauts food or specialties whose value can support its process costs.
Several vacuum systems have been used in conventional equipment, most of them consisting in a desublimator where water vapor is retained in a cooled wall and mechanical pump systems such as Root pumps and/or rotary or piston pumps.
Many apparatus and processes referred in U.S. Pat. Nos. 5,596,814; 5,269,077; 5,230,162; 5,131,168; 4,590,684; 4,035,924; 3,740,860 and 3,077,036 amongst others, have been developed to reduce FD costs, and its application has expanded to medium prize food as soluble coffee or tea, shrimps, some vegetables, soups, sauces, etc.
Freeze Drying technology can be classified in batch or continuous processes:
In batch processes, (see FIG. 1) the produces are frozen (Segment AB, FIG. 2) and placed in a drying chamber, where vacuum of around 1 Torr is applied (Segment BCD, FIG. 2). Heat is then transferred to the produce and sublimation of water occurs. As water is being eliminated, Freeze Drying velocity drops (FIG. 3) and produce's temperature raises slowly (Segment CD FIG. 2). When 0° C. is reached, Freeze Drying phase is finished, but in most cases there remains too much water to guarantee a long storage time at room temperature, so a desorption phase must take place, where water is eliminated up to a 3-10% residual content (Segment DE, FIG. 2).
In the continuous process, the produce passes through a series of chambers isolated one from the other by means of vacuum barriers, wherein the different stages of curve ABCDE of FIG. 2 take place.